The present invention relates generally to methods and apparatus for separating polarization modes of an input optical signal, rotating the polarization modes, then recombining the rotated polarization modes.
Light is a vector field that has two primary and orthogonal polarization states or vector directions. Generally, the polarization states are referred to as the S and P polarizations in free space optics, or the TE (Transverse Electric) and TM (Transverse Magnetic) modes of optical waveguides. The performance of optical waveguides and optical devices is often sensitive to the polarization state. That is, the response of the device changes as the polarization state changes. This is particularly pronounced in integrated optical waveguides that are fabricated on dielectric substrates.
Many optical components are insensitive to the input state of polarization. In fiber optic telecommunications, the polarization state of an optical signal that has traveled down any length of fiber is unknown, random, and time varying (due to perturbations in the environment). However, many devices remain polarization sensitive to some degree, and this affects ultimate performance, yield, and cost.
There are some applications where the TE and TM polarization states of an input optical signal need to be spatially split so each can be manipulated independently, such as, for example, PMD (Polarization Mode Dispersion) compensators, where the dispersion of the signal on the two states needs to be equalized. However, when an input light signal is split, or when a signal is rotated, there is usually error introduced into the signal due to inefficiencies in the polarization beam splitter or in the polarization rotator. The error comprises inefficiently split TE and/or TM modes, or inefficiently rotated TE or TM modes.
An apparatus and method for substantially removing the error components of an optical signal from the primary orthogonal components would be desirable.